This invention relates to a device used in submarine acoustics as a projector for emitting and as a hydrophone for receiving elastic waves. More particularly, the invention relates to a reversible electroacoustic transducer device having a constant directivity characteristic over a wide band of frequencies.
It is appropriate at this juncture to recall that the directivity of a transducer is the ability which this transducer has to distribute in a certain manner in space the energy which it exchanges with the propagation medium.
Problems are involved in the effective operation of surveillance listening systems having a wide band of signals propagated in water over the entire horizon. It can easily be shown that, in systems of this type, the signal-to-noise ratio is improved when the space to be monitored is divided into sectors each covered by an electroacoustic assembly having a directive emission characteristic.
However, the directivity obtained by conventional means is a function of frequency and has to be defined for a mean frequency in the service frequency band with the following disadvantages:
at low frequencies, the sectors thus defined overlap too far because the directional patterns are widened, whereas by contrast, PA1 at higher frequencies, these sectors no longer overlap because the directional patterns are narrower.
As a result, the useful frequency band of these systems is seriously limited.
Accordingly, there is a need to design electroacoustic assemblies of which the directivity characteristics remain as constant as possible with the frequency situated within the service frequency band.
The invention relates primarily to the production of devices having omnidirectional radiation patterns intended for use in amplitude goniometry systems in submarine acoustics.
However, the devices according to the invention may be used in every case where a given directivity characteristic has to be obtained and maintained over a wide frequency band both in passive reception and in active emission and detection. One example of this latter activity is the wide-band emission intended to obtain information on the frequency response of a submerged object.
Hereinafter a radiating device based on a reversible electroacoustic transducer will be referred to as an "acoustic antenna" or "antenna", both terms which are commonly used by submarine acoustic engineers.
An acoustic antenna may be formed by a vibrating surface of predetermined shape and size, for example a circular piston, or by a network of identical vibrators of which the size is small by comparison with the wavelength and which therefore has an omnidirectional radiation characteristic. In these two cases, the directivity function, which is the variation in sensitivity in dependence upon the direction of the incident wave, depends upon the frequency-size product of the vibrator or network. For the majority of so-called "broadside array" antennae, the angular width 2.alpha. of the principal lobe with an attenuation of -3 dB (whence the term 2.alpha..sub.3) is approximately inversely proportional to that product. Thus, for an antenna having given dimensions in a frequency band of one octave, the width 2.alpha..sub.3 decreases by half between the lowest frequency and the highest frequency.
According to the prior art, therefore, the establishment of a constant directivity characteristic as a function of frequency consists in keeping the effective frequency-size product constant. For example, starting from an antenna formed by a linear network of several equidistant electro-acoustic transducers and providing for a determined directivity at the frequency f, it is possible to obtain a constant directivity characteristic at the frequencies 2f and 4f by dividing the network into three sections and by increasing the number of pick-up transducers to form three similar networks (as diagrammatically illustrated in FIG. 1 for the case where the number of transducers is 4).
Other methods may be used and a description of the principal techniques known to the expert may be found in particular in an Article by J. C. MORRIS and F. HANDS "Constant-beam width arrays for wide frequency bands" published in the Journal ACUSTICA, Vol. II, 1961, pages 341-347. These techniques lead to devices which necessitate the use of filters or delay lines combined with a plurality of pick-ups. In addition, disturbances appear in the service frequency band in the overlapping frequency zones of two adjacent sectors.
The device provided by the invention is an acoustic antenna of simple construction of which the directivity characteristic remains constant over a wide frequency band and which comprises a reduced number of electroacoustic transducers.